Purification of dicarboxylic aromatic acids

ABSTRACT

Dicarboxylic aromatic acids containing impurities such as aldehydes may be purified by treating the acids at an elevated temperature in the presence of a catalyst containing metallic rhenium whereby the impurities may be removed and the purified acids recovered.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of my copending application,Ser. No. 69,019 filed Aug. 23, 1979, now abandoned, more teachings ofwhich are included herein with reference thereto.

BACKGROUND OF THE INVENTION

Dicarboxylic aromatic acids such as phthalic acid, isophthalic acid andterephthalic acid may be prepared by an oxidation process in which mixedalkyl aromatics or particular xylenes are subjected to an oxidationreaction with heavy metal salts and bromine acting as catalysts. Forexample, paraxylene may be oxidized to terephthalic acid in the liquidphase with a catalyst consisting of cobalt acetate and hydrogen bromidedissolved in acetic acid. An oxygen containing gas such as air isinjected into the reactor which is maintained at an elevated temperatureof around 200° C. and superatmospheric pressures of about 10atomspheres. The amount of air which is injected into the reactor issufficient to maintain an excess of oxygen in the off-gas over thestoichiometric requirement. The liquid which is drawn from the reactorwill contain about 25 to about 35 percent solids. These solids containthe desired terephthalic acid as well as a contaminant or impurity inthe form of paracarboxybenzaldehyde, an intermediate in the formation ofterephthalic acid. As the reaction proceeds and the terephthalic acidcrystallizes out of the reaction medium, it entrains with it the solidparacarboxybenzaldehyde in the form of mixed crystals, thereby retainingthe further oxidation of impurity difficult to achieve.

In the past, the working up and purification of terephthalic acid haspresented considerable problems inasmuch as the aforesaidparacarboxybenzaldehyde is a difficult compound to remove by the usualpurification methods. In view of the greater solubility ofparacarboxybenzaldehyde in acetic acid relative to terephthalic acid,the amount of impurity in the terephthalic acid will be greatly reducedbut will still be in excess of the maximum amount allowable in the acid.Therefore, it is necessary to utilize a relatively complex purificationsystem including various steps such as oxidation, neutralization,recycling, washing, distillation, etc.

Several U.S. patents have disclosed methods for purifying acids, andparticularly aromatic dicarboxylic acids. For example, U.S. Pat. No.3,546,285 discloses a method for purifying aromatic dicarboxylic acidssuch as terephthalic acid by catalytically hydrogenating the acid whichis dissolved in a solvent at an elevated temperature. The catalytichydrogenation compound which is employed in this process comprises anoble or other metal of Group VIII of the Periodic Table such asplatinum, palladium, nickel, etc. Likewise, U.S. Pat. No. 3,151,154 isdrawn to a method for decolorizing phthalic acids or salts thereof whichhave been prepared by the nitric acid oxidation of diloweralkyl benzenesand loweralkyl benzoic acids by reducing the nitro compounds in a liquidpolar solvent in the presence of a solid Group VIII metal hydrogenationcatalyst and thereafter acidifying the resulting reduced product.

U.S. Pat. No. 3,607,921 utilizes a process for purifying terephthalicacid by contact, in the presence of carbon monoxide, with solidparticles of an adsorptive agent which possesses substantial carbonmonoxide sorption capacity, while U.S. Pat. No. 3,456,001 purifies theseacids by utilizing a support Group VIII noble metal catalyst such asplatinum, palladium, ruthenium, rhodium, iridium, and osmium.

Yet another patent which teaches a purification process is U.S. Pat. No.3,522,298. This patent teaches the purification of terephthalic acid bycontacting a vaporous mixture and a gaseous medium with a solid materialcomprising a Group VIII metal, said gaseous medium comprising eitherhydrogen or oxygen.

As will hereafter be shown in greater detail, it has now been discoveredthat it is possible to remove contaminants or impurities fromdicarboxylic aromatic acids in a relatively inexpensive and simplemanner, said method involving the catalytic decarbonylation ofcontaminants utilizing a catalyst comprising a metallic rheniumcontaining compound while effecting the reaction in a liquid phasewithout requiring the presence of either hydrogen or oxygen.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a process for the purification of dicarboxylicaromatic acids. More specifically, the invention is concerned with theprocess for removing contaminants such as intermediate products whichhave not been fully oxidized and which are present in dicarboxylicaromatic acids, the latter having been prepared by the direct oxidationof alkyl aromatic compounds.

Dicarboxylic aromatic acids such as phthalic acid, isophthalic acid andterephthalic acid will find a wide variety of uses in the chemicalfield. For example, phthalic acid is used in dyes, especially in thesynthesis of indigo, in the manufacture of phthaleins, variousfluorescein and eosin dyes, as well as in the manufacture of rhodaminesand pyronine dyes. In addition, it is also used in the manufacture ofvarious medicines and synthetic perfumes. Isophthalic acid is used inthe manufacture of polyester and polyurethane resins as well as acomponent of plasticizers. Terephthalic acid is used as a reagent foralkali in wool as well as in the production of synthetic resins, fibersand films by combination with glycols, said synthetic materials beingsold under the tradenames of Dacron, Mylar, Terylene, etc. However, inorder to utilize these acids, it is necessary that the same be in arelatively pure state and not contain an inordinant amount ofcontaminants or impurities.

The present process involves a method for purifying dicarboxylicaromatic acids by decarbonylation of aldehyde impurities as exemplifiedby paracarboxybenzaldehyde, by utilizing a metallic rhenium containingcatalyst. The present process differs from the prior art which involvedthe removal of impurities from such acids by utilizing a hydrogenationreaction. In addition to using a hydrogenation process which involvedthe presence of Group VIII metal catalysts to effect the desiredreaction, it is also known that rhenium compounds and particularlyrhenium sulfide may be used as catalysts for hydrogenation reactions.However, rhenium sulfide cannot be used as a catalyst in the presentinvention which involves the use of metallic rhenium, inasmuch as sulfurpoisons metallic rhenium and therefore would render the metallic rheniumcontaining catalyst inoperative for effecting the desireddecarbonylation reaction.

It is therefore an object of this invention to provide a method for thepurification of dicarboxylic aromatic acids.

A further object of this invention is to provide a method for thepurification of dicarboxylic aromatic acids by treating said acids withcertain rhenium containing compounds.

In one aspect an embodiment of this invention resides in a process forthe purification of a dicarboxylic aromatic acid containing impuritiestherewith which comprises treating said acid in the presence of acatalyst comprising a rhenium containing compound at treatmentconditions, and recovering the purified dicarboxylic aromatic acid.

A specific embodiment of this invention is found in the process forpurification of terephthalic acid which comprises treating said acid inan aqueous medium in the presence of a catalyst comprising metallicrhenium deposited on carbon at a temperature in the range of from about150° to about 350° C. and in the presence of a nitrogen atmosphere andrecovering the purified terephthalic acid.

Other objects and embodiments will be found in the following furtherdetailed description of the invention.

As hereinbefore set forth, the present invention is concerned with aprocess for the purification of dicarboxylic aromatic acids, andparticularly the isomeric phthalic acids. The purification of theseacids to remove contaminants or impurities such as partially oxidizedcompounds as exemplified by aldehydes is effected by treating theimpurities containing acid in an aqueous medium in the presence of acatalyst which contains metallic rhenium. The purification of the acidis effected at elevated temperatures which may range from about 150° upto about 350° C. or more. In addition, the purification may also beeffected by pressures which may range from about 1 to about 100atmospheres. If superatmospheric pressures are employed, they may beafforded by the introduction of a gas into the reaction vessel, said gasbeing inert in nature such as argon, helium, etc.

The catalytic composition of matter which is utilized to effect thereaction will comprise a metallic rhenium containing compound, and if sodesired, this metallic rhenium compound may be dispersed on a solidmaterial. Some specific examples of solid supports which are utilizedinclude refractory oxides such as alumina, silica, mixtures ofrefractory oxides such as alumina-silica, alumina-zirconia,silica-zirconia, alumina-zirconia-magnesia, etc, silicon carbide,carbon, etc. In addition, if so desired, the catalyst may also bemodified to include at least one element selected from Group VIII of thePeriodic Table, said Group VIII metal being utilized to increase thecatalytic activity and stability of the catalyst. Examples of Group VIIImetals which may be employed will include in particular the noble metalssuch as platinum, palladium, ruthenium, osmium and iridium. It is alsocontemplated within the scope of this invention that the non-noblemetals such as iron, nickel, and cobalt may also be employed. However,it is to be understood that the catalytic activities to effect thedecarbonylation of the impure components which are present in thedicarboxylic aromatic acid is afforded by the metallic rhenium, and thatthe Group VIII metals are present merely as modifiers.

The process of this invention may be effected in any suitable manner andmay comprise either a batch or continuous type of operation. Forexample, when a batch type of operation is to be employed, a quantity ofthe particular dicarboxylic aromatic acid which is to be purified isplaced in an appropriate apparatus along with the metallic rheniumcontaining catalyst and water. Inasmuch as the reaction is to beeffected at superatmospheric pressures, the preferred type of vesselwhich is employed will comprise one that is pressure resistant such asautoclaves of the rotating, stirring, or mixing type, etc. The vessel isthen sealed and a gas which may be inert or reducing in nature ischarged to the vessel until the desired operating pressure is attained.Thereafter, the vessel and contents thereof are heated to the desiredoperating temperature and maintained thereat for a predetermined periodof time which may range from about 0.5 up to about 10 hours or more induration. Upon completion of the desired residence time, heating isdiscontinued and after the vessel has returned to room temperature theexcess pressure is discharged. The vessel is then opened and thereaction mixture is recovered therefrom, the purified acid beingseparated from the catalyst and water by conventional means such asfiltration, decantation, fractional distillation, etc.

It is also contemplated within the scope of this invention thatpurification of the dicarboxylic aromatic acid may be effected in acontinuous manner of operation. When this type of operation is employed,the impure acid is continously charged to a reactor which is providedwith the particular catalytic composition of matter utilized in thetreatment, said reactor being maintained at the proper operatingconditions of temperature and pressure hereinbefore set forth. The waterwhich supplies the aqueous medium in which the reaction is effected mayalso be continuously supplied to the reactor through a separate line or,if so desired, it may be admixed with the acid and the slurry chargedthereto in a single stream. Upon completion of the desired residencetime, the reactor effluent is continuously withdrawn from the reactorand subjected to conventional means of separation of the typehereinbefore set forth whereby the purified acid may be separated andrecovered.

The following examples are given for purposes of illustrating theprocess of this invention. However, it is to be understood that theseexamples are illustrative of the process and that the invention is notnecessarily limited thereto.

EXAMPLE I

A catalyst for the treatment of dicarboxylic aromatic acids to removeimpurities therefrom was prepared by impregnating 500 cc of silica gelwith a solution consisting of 350 ml of water, 10 cc of hydrochloricacid, 56.75 ml of palladium chloride containing 30 mg of palladium/mland 119.17 ml of ammonium perrhenate containing 25 mg of rhenium/ml,said impregnation being accomplished in a rotary evaporator. Afterimpregnation the catalyst was removed from the rotary evaporator anddried at a temperature of 120° C.

The purification of terephthalic acid was accomplished by treating 10grams of terephthalic acid which contained 7800 ppm ofparacarboxybenzaldehyde along with 2 grams of the catalyst preparedaccording to the above paragraph and 100 ml of water in a rotatingautoclave. The autoclave was sealed and air in the autoclave wasreplaced with nitrogen at an initial operating pressure of 1 atmosphere.The autoclave was then heated for a period of 1 hour at a temperature of250° C. At the end of this period heating was discontinued and after theautoclave was returned to room temperature the pressure was vented andthe reaction mixture was recovered. It was determined by polarographythat the terephthalic acid after treatment contained 200 ppm ofparacarboxybenzaldehyde.

EXAMPLE II

In the example, ammonium perrhenate was dissolved in water in a rotaryevaporator to form 20 ml of a solution containing 100 mg of rhenium/mlof solution. Following this, 100 cc of carbon was added to the solutionand impregnated with the ammonium perrhenate solution at steamtemperature. After impregnation, the sample was removed from the rotaryevaporator and dried at 120° C. Following this the catalyst was reducedat a temperature of 400° C. for a period of 2 hours in a stream ofhydrogen to form metallic rhenium.

Purification of the acid was accomplished by treating 100 grams of crudeterephthalic acid containing 7800 ppm of paracarboxybenzaldehyde with 1gram of the catalyst and 100 cc of water in an autoclave under 1atmosphere of nitrogen for a period of 1 hour at 250° C. At the end ofthis period heating was discontinued and after opening the autoclave thereaction mixture was filtered and dried at 120° C. Analysis of thesample by polarography showed that the acid contained 2500 ppm ofparacarboxybenzaldehyde.

EXAMPLE III

In this example, 1 gram of the metallic rhenium catalyst which wasprepared according to Example II above was ground to 8/20 mesh and usedto treat 10 grams of crude terephthalic acid containing 7800 ppm ofparacarboxybenzaldehyde (PCB). The acid and catalyst along with 100 mlof water was treated at a temperature of 250° C. for a period of 1 hour.At the end of this period the sample was dissolved in the water phase byadding an aqueous solution of sodium carbonate. Following this, thesolution was filtered and converted into terephthalic acid by theaddition of an aqueous solution of hydrochloric acid. The solidterephthalic acid was washed with water and dried at a temperature of120° C. for a period of 1 hour. Analysis of the sample showed a PCBlevel of 1400 ppm. In addition, chemical analysis of the treated sampledisclosed that it did not contain any rhenium.

When the catalyst was ground to a particle size of less than 100 meshand used to treat 10 grams of crude terephthalic acid similar in natureto that described in the above paragraph under similar conditions,analysis of the treated sample disclosed a PCB level of 130 ppm.

EXAMPLE IV

A catalyst for the treatment of dicarboxylic aromatic acids was preparedby dissolving 14.35 ml of palladium chloride containing 30.7 mg ofpalladium/ml, 22.10 ml of ammonium perrhenate containing 100 mg ofrhenium/ml and 5 ml of concentrated hydrochloric acid in 100 ml of waterin a rotary evaporator. Thereafter 100 cc of charcoal having an apparentbulk density of 0.442 was added to the solution and impregnation wasaccomplished at steam temperature. Following impregnation, the catalystwas then dried at 120° C. for a period of 16 hours. Following thedrying, the catalyst was then reduced in a stream of hydrogen at atemperature of 400° C. for a period of 2 hours to form metallic rhenium.

Purification of crude terephthalic acid containing 7800 ppm of PCB waseffected by treating 10 grams of the acid with 1 gram of the catalystwhich contained 1% palladium and 5% of metallic rhenium on the carbonalong with 100 ml of water for a period of 1 hour at a temperature of250° C. At the end of the reaction time, the sample was dissolved in thewater phase by adding an aqueous solution of sodium carbonate. Afterfiltration, an aqueous solution of hydrochloric acid was added and thesample was converted to terephthalic acid. After washing the sample withwater and drying at a temperature of 120° C. for a period of 1 hour,analysis disclosed that the purified terephthalic acid contained onlyabout 100 ppm of PCB.

When 1 gram of the catalyst prepared according to the example was groundto less than 100 mesh and used to treat 10 grams of crude terephthalicacid containing 7800 ppm PCB in a manner similar to that set forth inthe above paragraph, it was found that the PCB level of the treated acidwas less than 10 ppm and in addition, the treated sample did not containany rhenium.

EXAMPLE V

In this example, a catalyst similar in nature to that set forth inExample IV above was prepared by impregnating 100 cc of charcoal with asolution consisting of 100 ml of water, 5 ml of concentratedhydrochloric acid, 7.74 ml of ammonium perrhenate and 14.39 ml ofpalladium chloride. The catalyst after impregnation, drying andreduction contained 1% palladium and 1.75% of metallic rheniumcomposited on carbon.

When 1 gram of this catalyst was used to treat 10 grams of crudeterephthalic acid containing 7800 ppm PCB in 100 ml of water in a mannersimilar to that set forth in the above examples, the resultingterephthalic acid, after treatment thereof, was found to contain 105 ppmPCB.

When the catalyst was ground to less than 100 mesh and used in a manneridentical to that set forth above, the treated terephthalic acid wasfound to contain less than 10 ppm PCB as determined by polarography.

EXAMPLE VI

In this example, 14.21 ml of a platinum solution containing 31.1 mg ofplatinum/ml, 4.22 ml of ammonium perrhenate containing 100 mg ofrhenium/ml, and 5 ml of concentrated hydrochloric acid were dissolved in100 ml of water in a rotary evaporator. Thereafter 100 cc of charcoalwas added to the solution and impregnated at steam temperature. Theimpregnated sample was dried at a temperature of 120° C. for a period of16 hours and reduced in a stream of hydrogen at 400° C. for a period of2 hours to form metallic rhenium. Crude terephthalic acid containing7800 ppm PCB in an amount of 10 grams was treated with 1 gram of thiscatalyst along with 100 ml of water at a temperature of 250° C. for aperiod of 1 hour. The recovered sample was treated with sodium carbonateand hydrochloric acid in a manner similar to that set forth in the aboveexamples and the recovered terephthalic acid was found to contain 92 ppmPCB by polarography.

When 1 gram of the catalyst prepared according to this example wasground to less than 100 mesh and used to treat crude terephthalic acidin a similar manner, the treated sample was found to contain 220 ppmPCB.

EXAMPLE VII

In this example, a catalyst which contained 1% palladium and 5% metallicrhenium was prepared in a manner similar to that set forth in Example VIabove. The treatment of crude terephthalic acid containing 7800 ppm ofPCB was accomplished by utilizing 1 gram of catalyst particles and 100ml of water. A sample was treated in a manner similar to that set forthin the above examples and after recovery was found to contain 88 ppm ofPCB.

When the catalyst was ground to less than 100 mesh and used to treat asimilar sample of terephthalic acid, the treated acid after recovery wasfound to contain 48 ppm PCB.

I claim as my invention:
 1. A process for the purification of an acidselected from the group consisting of phthalic, isophthalic, andterephthalic acids containing paracarboxybenzaldehyde as an impurity,which comprises contacting said acid in an aqueous medium with acatalyst comprising metallic rhenium at a temperature of from about 150°to about 350° C. to effect decarbonylation of saidparacarboxybenzaldehyde and recovering the thus purified acid.
 2. Theprocess as set forth in claim 1 in which said purification is effectedin an inert atmosphere.
 3. The process as set forth in claim 2 in whichsaid inert atmosphere is afforded by nitrogen.
 4. The process as setforth in claim 1 in which said metallic rhenium is deposited on a solidsupport.
 5. The process as set forth in claim 4 in which said metallicrhenium is deposited on carbon.
 6. The process as set forth in claim 4in which said catalyst comprises metallic rhenium deposited on alumina.7. The process as set forth in claim 1 in which said acid is phthalicacid.
 8. The process as set forth in claim 1 in which said acid isisophthalic acid.
 9. The process as set forth in claim 1 in which saidacid is terephthalic acid.